The operating conditions of the vehicle batteries consist of variable envi- ronmental conditions and variable electric power demands. Chemical processes in the battery are temperature dependent. Therefore, the elec- trochemical storage system will have to be kept within certain temper- ature limits in order to maintain a proper function and also to ensure a reasonable battery life. The temperature limits for the battery are con- sidered to be 10°C as the lower limit due to decreasing battery capacity and 50°C as the upper limit due to positive plate corrosion and separa- tor decomposition.
From the battery thermal management standpoint, it is necessary to maintain a uniform temperature within the battery pack. The thermal management system will provide either heating or cooling action depending upon the battery pack conditions. Tests conducted in the lab- oratory and with EV urban driving suggest that using a thermal man- agement system improves the mileage and battery life by at least 20%.
Thermal management of the battery pack is essential both for normal urban driving and rapid overnight charging where charger current levels of hundreds of amperes are applied to the battery pack for relatively short duration.
Although maintenance-free starved electrolyte or gelled-electrolyte VRLA batteries are being used commonly, these batteries overheat more rapidly than the flooded lead-acid counterparts. This is owing to the fact that the VRLA batteries are unable to dissipate heat generated by gas
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recombination during the charging and I2R effects during discharging processes.
The primary design of a thermal management system should keep the battery sufficiently insulated. The insulation will assist to obtain an acceptably high-operating temperature during winter and cool during summer by means of an air flow during the charging time. The battery remains in the battery pack during the charging process. The secondary design criterion is that the circulation and the cooling air flow should be properly distributed in space in order to ensure a minimum temper- ature difference between the individual battery modules. This design cri- terion should be applicable under various operating conditions.
The thermal capacity of the battery module is derived by calculation and by measurements. The manufacturer-provided information avail- able concerning materials and dimensions is used to calculate the battery pack thermal capacity.
The heat generation of the battery modules is calculated using a simulated driving profile: 185 A in 15 seconds, 61.7 A in 25 seconds, and 0 A in 30 seconds. The charge is performed at 18.5 A until a cell voltage of 2.35 V/cell is reached. After the 2.35 V/cell charge voltage is obtained, the battery is maintained at 9.25 A for 4 hours. The normal discharge lasts for 3 hours, and the normal charge lasts 8 hours plus 4 hours.
The heat generated during the electrical cycling is measured in an isothermal air-flow calorimeter at different temperature levels. The dis- sipation during discharge is dependent on the temperature level. Start- ing at approximately 50 W at 50°C decreasing to 25 W at 40°C. The heat dissipation is dependent on the state of discharge with a peak discharge in the beginning, a minimum and a considerable increase in the dis- sipation at the end of the discharge.
The battery module container analysis accounts for different envi- ronmental and operating conditions. The model of the module consists of four parts, a heat transfer model, a heat generation model, an ambient temperature model, and a vehicle operation model. The air-flow model is made by the subdivision of the battery module air-flow stream into several individual sections—10 different standard sections. Each section is characterized by its geometrical properties and the calculations are simplified in order to obtain values for the kinetic and the viscous con- tribution to the air-flow resistance. The viscous forces are approximated considering a laminar battery air flow in a channel of two parallel plates.
The kinetic forces during the air flow are approximated as fully lost by all discontinuities in the battery pack. Each battery flow section is char- acterized by two constants, k1and k2, expressing the individual section 138 ELECTRIC VEHICLE BATTERY PERFORMANCE
pressure difference Dp as a function of the air stream q. The air-flow dif- ference is expressed as
Dp =k2¥q2+k1¥q
The steady state solution of all the sections is calculated by a computer, using a computer analyses program.
The heat transfer model takes into account the internal heat trans- port. This heat transfer is due to conduction of heat from the section surfaces through the air over the half air channel width to the air stream.
It also accounts for the movement of the heated air by the air stream expressed as a fluid conductance, F as a function of q, the air density r the air specific thermal capacity cp.
The fluid conductance and the radiation between the batter modules is referred to as F = q ¥ r ¥ cp. The heat transfer also contains con- duction through the container walls, free convection from the outer surfaces, and the radiation to the battery pack surroundings. The heat dissipation and the heat capacity of the battery module are also included.
The heat generation model for the battery module takes into account the battery temperature and the charge/discharge status. The relation- ship is selected as a best-fit linear approximation based on results obtained from the isothermal air-flow calorimeter measurements.
The ambient temperature model describes the ambient temperature as a function of the time of the day. As well as constant temperature as different day/night temperature profiles corresponding to the different seasons.
The vehicle operational model describes the use of the vehicle during the day. It also considers two drive periods—one in the morning and one in the afternoon—and an overnight charge period. Two different modes of use have been considered, one implying the full driving range of the vehicle and one of half the range.
The combined thermal model of the battery pack is used to calculate the temperature response of the battery in the time domain under various conditions. The results were analyzed by considering tempera- ture levels and temperature differences.
Two commonly used thermal management systems are the circulat- ing-air and the circulating-liquid systems. Using either method of thermal management, it is important to maximize performance, mini- mize cost, complexity, weight, and power requirements. The circulating- air management system can be constructed rather inexpensively around the existing battery pack. As a variation to the circulating-air system, the
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battery pack may be divided into several sections with individual fans circulating the air in the battery pack. This variation allows for maximum surface exposure and allows adequate air flow around, over, under each battery in the pack. This system adds very little additional weight and requires additional power for the fan and the heater.
However, it may be difficult to maintain the temperature gradients to and from the air and the battery pack.
A circulating-liquid thermal management system, provides better cooling and heating than the circulating air system. A liquid solution of ethylene glycol and water as a coolant provides for good heat exchange and reduced volume requirements. The system will become more complex as the heat transfer rates increase. The system is heavier, complex, and expensive compared to the circulating air thermal management system. In addition to the additional weight, the power requirements are more due to additional components like pump and filter.
A complex circulating-liquid thermal management system may include water and ethylene glycol coolant solution jackets between the individual batteries. In addition, it may be necessary to provide for monitoring and thermal management of individual battery cells.